1. Field of the Invention
The present invention relates to radio communication and, more particularly to a radio data transmission system having great effects in application to vehicle or similar mobile radio communication where a problem of complicated reflected waves is existent.
2. Description of the Prior Art
In the mobile radio communication, input waves are in most cases composed of reflected waves from various buildings and so forth. And both the amplitude and the phase of the received waves, which are composite signals of such reflected waves, are varied intricately in accordance with movement of a mobile station.
FIGS. 1 and 2 illustrate how such variations are caused. In FIG. 1 are shown a base station S, a mobile station M, two reflection points R1 and R2, and two different wave routes a and b. Represented by Ea and Eb are the amplitudes of input waves received respectively from the route a via the reflection point R1 and the route b via the reflection point R2. Since the reflection points R1 and R2 are shifted with movement of the mobile station M, the phases of the input waves received via the routes a and b respectively are also changed to consequently cause variations in both amplitude and phase of the composite waves as shown by waveforms (A) and (B) in FIG. 2. During an interval III in FIG. 2, Ea and Eb are substantially equal to each other in relative amplitude and are substantially opposite to each other in relative phase. (For convenience of explanation, such interval is chosen as III.) Therefore the composite amplitude is widely varied to eventually bring about a sharp variation in the composite phase, hence inducing inversion thereof. During intervals I and II anterior and posterior respectively to such variation, there occurs relatively small variation in both amplitude and phase.
In phase modulation digital communication where data is transmitted on the basis of a phase difference obtained with reference to the phase of a preceding bit, if the amplitude and the phase of the received composite signal is sharply varied as shown in the interval III, it becomes difficult to detect the phase difference from the preceding bit, hence inducing ready generation of code error. Such sharp variations of the amplitude and the phase occur substantially at every half wavelength in most cases although being dependent also on the positional relationship between the reflection points R1, R2 and the mobile station M. For example, under the conditions including a vehicle speed of 40 km/hr and a frequency of 900 MHz (half wavelength about 17 cm), it follows that sharp amplitude and phase variations occur substantially at every 15 msec (.apprxeq.17/40.times.10.sup.5 .times.1/60.times.60). Supposing now the length of interval III is at a ratio of about 1/10 to the entirety, data of 1.5 msec fails to be transmitted properly to generate extremely great code error as a result. In order to eliminate such a problem, it has been customary heretofore to form an error correction code of a high redundancy so that any error resulting from some data dropouts can be corrected.
In the conventional radio data transmission system where it is inevitable to use such error correction code of an extremely high redundancy as mentioned, the rate of the essential transmission data is low in the gross content and therefore the data transmission speed is obliged to be considerably lowered due to the restriction of radio frequency assignment. In addition, there are further problems in practical use including the necessity of a large-sized and complicated decoding apparatus.